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Alternator Wiring - TD5 instruments with P38 Alternator


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I've been looking at this for some hours now and I'm not sure it's going to end without a fire, so I need some help.

I know alternator wiring has been raised here many times, but this is slightly different.

I have a P38 alternator with 3 contacts on the rear, standard stuff. Stole this photo to demonstrate:

post-23013-0-46850100-1363368149.jpg

Left hand is the main output, D+ (middle) is the alternator to instruments connector and W (right) is for the rev counter.

This is the TD5 wiring diagram. Three contacts, two the same as above, but no W due to the ECU-controlled rev counter, leaving one wire with nothing to connect to:

image.png.a825ca8cfe1ec6a1c7f6e9981aba1314.png

 

image.png.2c3fac4bce78c40ead32f6c2006586f8.png

So do I:

  •  dispense with the WG wire altogether
  • put both onto the D+ terminal
  • Put the WG wire on the W terminal
  • forget about having a working charge warning light with this alternator
  • Stop posting questions at 1am?

I would just go and test this myself, but I don't have a completed instrument harness yet.

I get that the lamp illuminates when there's a differential between the two, so surely they can't go on the same terminal? Really hoping I can keep this alternator for its 100amp goodness, but open to ideas.

 

Edited by ThreePointFive
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A bit more info please.

What car? I guess Disco2, as the Defender TD5 didn't have a rev counter as standard. If it's for an aftermarket rev counter in a Defender, just use a wire directly from the W-post to the gauge. That's what we did on my TD5 90, because the ECU rev signal would sometimes drop while winching.

And what engine/ECU? GEMS/MS? There's a good chance you'll be able to use an output from the ECU to drive the gauge.

Filip

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The dash warning light to the middle terminal, main cable to battery/starter to the left connection & rev counter to the right connection. 

the White/green trace could go on the same connection as the main cables, 

 

Quote

Stop posting questions at 1am?

that's good idea :rolleyes:

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4 minutes ago, western said:

The dash warning light to the middle terminal, main cable to battery/starter to the left connection & rev counter to the right connection. 

that's good idea :rolleyes:

Well Ralph, that last part certainly changes the tone that post can be read with. I hope your day gets better.

 

The question I am asking - as badly worded as it may have been - from my understanding of the TD5 system is that essentially have two 'dash warning light' feeds, which go to different contacts on the TD5 alternator. I don't know if the W terminal can act as that 'extra' terminal, I suspect not given it has one job to do, but someone who understands more about the inner workings of these things might have be able to assist.

Put rev counters to one side for the moment, that's not the issue in this case.

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I’m not sure if this is of any help but I disconnected the alternator on my Thor 4.0 yesterday and noticed it only had two connections one heavy brown to the battery and a thin brown and yellow to terminal D+.

If you can trace the WG wire that isn’t required and either strip it out or disconnect it then it should be safe.

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45 minutes ago, ThreePointFive said:

Well Ralph, that last part certainly changes the tone that post can be read with. I hope your day gets better.

 

The question I am asking - as badly worded as it may have been - from my understanding of the TD5 system is that essentially have two 'dash warning light' feeds, which go to different contacts on the TD5 alternator. I don't know if the W terminal can act as that 'extra' terminal, I suspect not given it has one job to do, but someone who understands more about the inner workings of these things might have be able to assist.

Put rev counters to one side for the moment, that's not the issue in this case.

Sorry it was meant to be light humoured, 

I was thinking that if the white/green goes to the B+ terminal, that would still give a feed to the 10amp fuse & the ignition switch,  the Td5 alternator is a battery sense unit not a machine sense like the older vehicles &  the alternator you have.

 

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2 hours ago, western said:

Sorry it was meant to be light humoured, 

I was thinking that if the white/green goes to the B+ terminal, that would still give a feed to the 10amp fuse & the ignition switch,  the Td5 alternator is a battery sense unit not a machine sense like the older vehicles &  the alternator you have.

 

Well, you weren't wrong, I shouldn't be posting at that time of night! It's frustrating because it's a question that must have been addressed by others doing the same conversion as me, but I'm the only one that apparently can't work it out for myself.

I think the best thing for me to do is just run it and if what lights up and goes out depending on connections to the B+.

Edited by ThreePointFive
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Spot on Paul, thanks. I've re-purposed the other wire as the W terminal feed for the rev counter when I get one, so even that is useful now (running to a different connector).

 

Must be the 5th time you've saved my build from myself.

Edited by ThreePointFive
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